Propagation velocity tuning with functionalized carbon nanomaterial in printed wiring boards (PWBs) and other substrates, and design structures for same

ABSTRACT

An apparatus has a permittivity attenuation layer interposed between a substrate and a first conductive trace, wherein the permittivity attenuation layer comprises a resin matrix containing functionalized carbon nanomaterial, such as functionalized single-wall carbon nanotubes (f-SWNTs). In some embodiments, a design structure for designing, manufacturing, or testing the apparatus is tangibly embodied in a machine readable medium. In some embodiments, the apparatus comprises an enhanced laminate core for use in a printed wiring board (PWB) that contains a differential pair having an inner-leg conductive trace and an outer-leg conductive trace. A permittivity attenuation layer is interposed between the inner-leg conductive trace and a laminate core, wherein the loading level of f-SWNTs in the permittivity attenuation layer is selected to attenuate the permittivity of the inner-leg conductive trace to match the permittivity of the outer-leg conductive trace.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a divisional application of pending U.S.patent application Ser. No. 14/730,286, filed Jun. 4, 2015, entitled“PROPAGATION VELOCITY TUNING WITH FUNCTIONALIZED CARBON NANOMATERIAL INPRINTED WIRING BOARDS (PWBs) AND OTHER SUBSTRATES, AND DESIGN STRUCTURESFOR SAME”, which is a continuation application of pending U.S. patentapplication Ser. No. 14/674,011, filed Mar. 31, 2015, entitled“PROPAGATION VELOCITY TUNING WITH FUNCTIONALIZED CARBON NANOMATERIAL INPRINTED WIRING BOARDS (PWBs) AND OTHER SUBSTRATES, AND DESIGN STRUCTURESFOR SAME”, each of which is hereby incorporated herein by reference inits entirety.

BACKGROUND

The present invention relates in general to printed wiring boards (PWBs)and other substrates, and to design structures for same. Moreparticularly, the present invention relates to tuning the propagationvelocity of a signal through a conductive trace of a PWB or othersubstrate by interposing a permittivity attenuation layer containingfunctionalized carbon nanomaterial, such as functionalized single-wallcarbon nanotubes (f-SWNTs), between the conductive trace and thesubstrate, and to a design structure for same.

SUMMARY

According to some embodiments of the present invention, an apparatus hasa permittivity attenuation layer interposed between a substrate and afirst conductive trace, wherein the permittivity attenuation layercomprises a resin matrix containing functionalized carbon nanomaterial,such as functionalized single-wall carbon nanotubes (f-SWNTs). In someembodiments of the present invention, a design structure for designing,manufacturing, or testing the apparatus is tangibly embodied in amachine readable medium. In some embodiments of the present invention,the apparatus comprises an enhanced laminate core for use in a printedwiring board (PWB) that contains a differential pair having an inner-legconductive trace and an outer-leg conductive trace. A permittivityattenuation layer is interposed between the inner-leg conductive traceand a laminate core, wherein the loading level of f-SWNTs in thepermittivity attenuation layer is selected to attenuate the permittivityof the inner-leg conductive trace to match the permittivity of theouter-leg conductive trace. Hence, in accordance with some embodimentsof the present invention, it is possible to eliminate, or at leastsubstantially reduce, in-pair skew.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the present invention will hereinafter be described inconjunction with the appended drawings, where like designations denotelike elements.

FIG. 1 is a flow diagram illustrating, through a sequence of stages1(a)-1(f), a method of fabricating an enhanced laminate core having apermittivity attenuation layer containing functionalized carbonnanomaterial, such as functionalized single-wall carbon nanotubes(f-SWNTs), interposed between a laminate core and an inner-legconductive trace of a differential pair in accordance with someembodiments of the present invention.

FIG. 2 is a chemical reaction diagram showing an exemplary synthesis offunctionalized single-wall carbon nanotubes (f-SWNTs) in accordance withsome embodiments of the present invention.

FIG. 3 is a chemical reaction diagram showing another exemplarysynthesis of functionalized single-wall carbon nanotubes (f-SWNTs) inaccordance with some embodiments of the present invention.

FIG. 4 is a flow diagram illustrating a design process used in printedwiring board (PWB), laminate subcomposite, interconnect substrate, flexcable, or semiconductor IC logic design, manufacture, and/or test inaccordance with some embodiments of present invention.

DETAILED DESCRIPTION

When a signal is propagated on a differential pair of conducive tracesrouted on a printed wiring board (PWB) at the board level (or on a chipat a chip level), each trace propagates a mirror image wave form. Thatis, a differential pair is two complementary conductive traces thattransfer equal and opposite signals down their length. Differentialpairs are also referred to as “differential trace pairs.”

A length mismatch is created when a differential pair is routed througha turn (i.e. the outer leg of the differential pair is longer than theinner leg of the differential pair), resulting in degraded signalintegrity and increased radiated electromagnetic interference (EMI). Forpurposes of this document, including the claims, the terminology“outer-leg conductive trace” refers to the conductive trace of adifferential pair that lies on the outside of a turn made by thedifferential pair (i.e., this is the “longer” conductive trace throughthe turn), and the terminology “inner-leg conductive trace” refers tothe conductive trace of that same differential pair that lies on theinside of the turn made by the differential pair (i.e., this is the“shorter” conductive trace through the turn).

Typically, electrical designers try to mitigate the impact of turns inthe routing of differential pairs by compensating left hand turns withan equal number of right hand turns, or via incorporation of one or more“trombones” on the shorter leg (i.e., the inner-leg conductive trace). Atrombone, which is a deviation from the most direct path, adds length toa conductive trace. A conductive trace that incorporates a trombonedeviates into surrounding available space and then loops back uponitself. The practice of using such loopbacks is referred to as“tromboning”. Additional details regarding trombones are disclosed inU.S. Pat. No. 6,349,402 B1.

Each of these conventional techniques for mitigating the impact of turnsin the routing of differential pairs (i.e., compensating left hand turnswith an equal number on right hand turns and “tromboning”) can result ina more complex layout and/or an impedance discontinuity. These and otherdeleterious effects introduced by trombones, in general and in thecontext of differential pairs, are disclosed in Brooks, “AdjustingSignal Timing (Part 1),” UltraCAD Design, Inc., 2003, pp 1-9.Consequently, a need exists for a mechanism to eliminate, or at leastsubstantially reduce, in-pair skew that does not depend on either of theabove-discussed conventional mitigation techniques (i.e., compensatingleft hand turns with an equal number on right hand turns and“tromboning”).

In accordance with some embodiments of the present invention, anapparatus has a permittivity attenuation layer interposed between asubstrate and a first conductive trace, wherein the permittivityattenuation layer comprises a resin matrix containing functionalizedcarbon nanomaterial, such as functionalized single-wall carbon nanotubes(f-SWNTs). For example, the apparatus may be in the form of a printedwiring board (PWB), a chip, or other substrate, such as a laminatesubcomposite, an interconnect substrate (e.g., an interposer or a modulesubstrate), or a flex cable. Printed wiring boards (PWBs) are alsoreferred to as printed circuit board (PCBs).

In some embodiments of the present invention, a design structure fordesigning, manufacturing, or testing such an apparatus is tangiblyembodied in a machine readable medium. These embodiments encompass theapparatus as it resides in design files or design structures (e.g.,GDSII, GL1, or OASIS data files).

In some embodiments of the present invention, the apparatus comprises anenhanced laminate core for use in a printed wiring board (PWB) thatcontains a differential pair having an inner-leg conductive trace and anouter-leg conductive trace. A permittivity attenuation layer isinterposed between the inner-leg conductive trace and a laminate core,wherein the loading level of f-SWNTs in the permittivity attenuationlayer is selected to attenuate the permittivity of the inner-legconductive trace to match the permittivity of the outer-leg conductivetrace. Hence, in accordance with some embodiments of the presentinvention, it is possible to eliminate, or at least substantiallyreduce, in-pair skew without depending on either of the above-discussedconventional mitigation techniques (i.e., compensating left hand turnswith an equal number on right hand turns and “tromboning”).

Dielectric constant (Dk) is an important parameter related to dielectricloss in PWBs. Dk is also referred to as relative permittivity. In a PWBlaminate, the Dk is the ratio of the capacitance between a pair ofconductors separated by a dielectric material (e.g., epoxy-based resin)compared to the capacitance between that pair of conductors in a vacuum.The Dk of a PWB laminate will vary depending on the PWB substratematerial used to make it, as well as signal frequency. PWB substratematerials with lower Dk values contribute to a lower dielectric loss.Moreover, in PWB laminates made from PWB substrate materials with higherDk values, signals will propagate more slowly through the conductors. Infact, the propagation delay time is a function of the square root of theDk value of the PWB substrate material.

As signal speeds increase (i.e., as signal frequency increases), theneed for PCB substrate materials having low Dk (e.g., at 1-3 GHz,Dk≦4.0, preferably Dk<3.7, more preferably Dk<3.5) becomes critical. Asa point of reference, the Dk of FR4 is approximately 4.3 at 1 GHz. FR4,which is a composite material composed of woven glass fabric impregnatedwith an epoxy resin varnish, is used in many PCBs.

Within this general environment of PCB substrate materials having lowDk, in accordance with some embodiments of the present invention,localized regions of higher Dk (i.e., the Dk of these localized regionsis “higher” relative to the Dk of the surrounding PCB substratematerials) are introduced to produce a predetermined amount of signalpropagation delay in portions of conductive traces routed through theseregions. These localized regions of higher Dk serve to attenuate thepermittivity of the portions of the conductive traces routed throughthese regions. Accordingly, these localized regions of higher Dk arereferred to herein as “permittivity attenuation layers.” The presentinvention takes advantage of a signal propagation delay effectintroduced by a permittivity attenuation layer interposed between aconductive trace and a substrate material, wherein the permittivityattenuation layer comprises a resin matrix containing functionalizedcarbon nanomaterial, such as functionalized single-wall carbon nanotubes(f-SWNTs). Optionally, the permittivity attenuation layer may containnon-functionalized (i.e., raw) carbon nanomaterial, such asnon-functionalized single-wall carbon nanotubes (SWNTs), in the resinmatrix in addition to the functionalized carbon nanomaterial.

Generally, the permittivity of polymer composite materials can beadjusted by the addition of carbon nanomaterial. Increasing the loadinglevel of carbon nanomaterial in polymer composite materials typicallyincreases the permittivity. For example, the real permittivity (Er) ofSWNT polymer composites, at 500 MHz-5.5 GHz, can be adjusted (increased)by a factor of approximately 35× by varying the loading level ofnon-functionalized SWNTs from 0 wt % to 23 wt %.

Typically, increasing the loading level of non-functionalized carbonnanomaterial in a polymer composite material increases the permittivityat a much faster rate than increasing the loading level offunctionalized carbon nanomaterial in that same polymer compositematerial. Functionalization of SWNTs, for example, disrupts theconducting network in the carbon nanotube and thereby dramaticallychanges permittivity and other electronic properties. Even a low degreeof functionalization (e.g., a SWNT is functionalized to contain a singlefunctional group per every 100 carbons) removes the metallic-like vanHove transitions in SWNTs. Hence, at a given loading level, thepermittivity of a polymer composite material containing functionalizedcarbon nanomaterial will be considerably lower than that same polymercomposite material containing non-functionalized carbon nanomaterial.Higginbotham et al., “Tunable Permittivity of Polymer Composites throughIncremental Blending of Raw and Functionalized Single-Wall CarbonNanotubes”, J. Phys. Chem. C, 2007, Vol. 111, pp 17751-17754, which ishereby incorporated herein by reference in its entirety, discloses thatby simply blending the two types of single-wall carbon nanotubes(f-SWNTs and raw SWNTs) together into the same silicone elastomer matrixat varying ratios, while keeping the total weight percent of SWNTsincluded in the resulting composite constant at 0.5 wt %, the realpermittivity (Er) of the resulting composite can be tuned to any desiredvalue between 20 and 3.

As noted above, the present invention takes advantage of a signalpropagation delay effect introduced by a permittivity attenuation layerinterposed between a conductive trace and a substrate material, whereinthe permittivity attenuation layer comprises a resin matrix containingfunctionalized carbon nanomaterial, such as f-SWNTs. Optionally, thepermittivity attenuation layer may contain non-functionalized (i.e.,raw) carbon nanomaterial, such as non-functionalized SWNTs, in additionto the functionalized carbon nanomaterial. Permittivity can be varieddepending on the degree of functionalization and/or loading level(s) offunctionalized carbon nanomaterial and non-functionalized carbonnanomaterial (if any) in the resin matrix of the permittivityattenuation layer.

The permittivity attenuation layer may be, for example, interposedbetween an inner-leg conductive trace of a differential pair and asubstrate to slow down signal propagation through the inner-legconductive trace. Preferably, the permittivity attenuation layer isinterposed between the inner-leg conductive trace of a differential pairand the substrate only in a length-mismatch region defined by the turnthrough which the differential pair traverses (i.e., thislength-mismatch region extends from a location near where the lengthmismatch between the outer- and inner-leg conductive traces begins to alocation near where the length mismatch ends). The turn through which adifferential pair traverses is often expressed in terms of an angle(e.g., 45°, 90°, and the like).

Hence, in accordance with some embodiments of the present invention, itis possible to vary the permittivity of the composite laminate materialto account for the angle through which such a differential pairtraverses. It is possible, therefore, to vary the permittivity of thecomposite laminate material to eliminate, or at least substantiallyreduce, in-pair skew. Moreover, in accordance with some embodiments ofthe present invention, the functionality of the carbon nanomaterial inthe resin matrix can be matched to the composite laminate material and,hence, the functionalized carbon nanomaterial becomes part of(covalently bonds to) the matrix.

The carbon nanomaterial that can be used (in non-functionalized and/orfunctionalized form) in accordance with some embodiments of the presentinvention may be either hollow (e.g., carbon nanotubes (CNTs)) or solid(e.g., carbon nanofibers (CNFs)). Carbon nanotubes include single-wallcarbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs).

Commercially available SWNTs that can be used (in non-functionalizedand/or functionalized form) in accordance with some embodiments of thepresent invention include, but are not limited to, HiPco SWNTs availablefrom Unidym Inc., Sunnyvale, Calif., USA.

Commercially available MWNTs that can be used (in non-functionalizedand/or functionalized form) in accordance with some embodiments of thepresent invention include, but are not limited to, VGCF and VGCF-H bothavailable from Showa Denko K.K., Tokyo, Japan and FloTube 9000 availablefrom CNano Technology, Limited, San Francisco, Calif., USA.

Commercially available CNFs that can be used (in non-functionalizedand/or functionalized form) in accordance with some embodiments of thepresent invention include, but are not limited to, Pyrograf-III(PR-19-XT-LHT) available from Pyrograf Products, Inc., Cedarville, Ohio,USA.

The loading level of the functionalized carbon nanomaterial (and,optionally, the loading level of the non-functionalized carbonnanomaterial) in the resin matrix of a permittivity attenuation layer,in accordance with some embodiments of the present invention, may beempirically determined based on the amount of permittivityattenuation/signal propagation delay that is desired. Typically, thetotal loading level of both the functionalized and non-functionalizedcarbon nanomaterial will be within the range from 0.1 wt % to 25 wt %.The loading level of the functionalized carbon nanomaterial mayrepresent any portion or the entirety of the total loading level.

In accordance with some embodiments of the present invention, a wiringdiagram (e.g., showing the routing of the individual conductive tracesof differential pairs of a PWB, a laminate subcomposite, or othersubstrates) may be used to determine where the functionalized SWNTs(and/or other functionalized carbon nanomaterial and, optionally,non-functionalized carbon nanomaterial) need to be incorporated. Thefunctionalized SWNTs (and/or other functionalized carbon nanomaterialand, optionally, non-functionalized carbon nanomaterial) may beincorporated where needed (e.g., underlying an inner-leg conductivetrace of a differential pair in a length-mismatch region defined by theturn through which the differential pair traverses) via conventionallithography techniques. FIG. 1 illustrates one such embodiment.

FIG. 1 is a flow diagram illustrating, through a sequence of stages1(a)-1(f), a method 100 of fabricating an enhanced laminate core havinga permittivity attenuation layer containing functionalized carbonnanomaterial, such as functionalized single-wall carbon nanotubes(f-SWNTs), interposed between a laminate core and an inner-legconductive trace of a differential pair in accordance with someembodiments of the present invention. In FIG. 1, each of the stages1(a)-1(f) is a partial, sectional view of the same section at successivestages of fabrication of the enhanced laminate core, which is to befurther processed to produce a PWB.

In the method 100, the steps discussed below (steps 105, 110, 115, 120and 125) are performed. These steps are set forth in their preferredorder. It must be understood, however, that the various steps may occursimultaneously or at other times relative to one another. Moreover,those skilled in the art will appreciate that one or more steps may beomitted.

Stage 1(a) of FIG. 1 shows a partial, sectional view of a laminate core102. The laminate core 102 may be, for example, constructed of a glasscloth or other glass fiber substrate impregnated with a varnish coating.The glass cloth is typically constructed of bundles of glass fiberswoven together in an orthogonal fashion, with the bundles typicallybeing perpendicular to each other. The varnish coating may be anysuitable resin, including but not limited to, an epoxy-based resin, abismaleimide triazine (BT) resin, a polyphenylene oxide(PPO)/triallylisocyanurate (TAIC) composition, and combinations thereof.Suitable epoxy-based resins include, but are not limited to, bisphenol-Atype epoxy resins, polyglycol di-epoxide liquid resins, and the like.Bisphenol A type epoxy resins, which are produced from bisphenol A andepichloroydrin, are commercially available. Depending on theapplication, flexible resins such as polyglycol di-epoxide liquid resinsmay be preferred to impart flexibility. Polyglycol di-epoxide liquidresins, which are produced from polypropylene glycol andepichlorohydrin, are commercially available.

The method 100 begins by laminating a photoresist (104 shown in stage1(b), described below) to the laminate core 102, followed byconventional expose and develop processes to open up various regions(e.g., region 108 shown in stage 1(b), described below) on the laminatecore 102, including those regions on the laminate core 102 that willrequire permittivity modification (step 105).

Stage 1(b) of FIG. 1 shows a partial, sectional view of the laminatecore 102 having a photoresist 104 laminated thereon and having a region108 opened up in the photoresist 104 by conventional expose and developprocesses. The open region 108 corresponds to a region where aninner-leg conductive trace (120 shown in stage 1(f), described below) ofa differential pair (122 shown in stage 1(f), described below) will beformed and where permittivity modification is required via formation ofa permittivity attenuation layer (114 shown in stage 1(c), describedbelow). Hence, the open region 108 is also referred to herein as the“permittivity attenuation layer formation region.”

The open region 108 requires permittivity modification, but an adjacentregion where an outer-leg conductive trace (118 shown in stage 1(f),described below) of the differential pair 122 will be formed does notrequire permittivity modification. Accordingly, the permittivityattenuation layer 114 containing functionalized carbon nanomaterial,such as f-SWNTs, (and, optionally, non-functionalized carbonnanomaterial, such as non-functionalized SWNTs) is applied to the openregion 108. Henceforth in the description of method 100, f-SWNTs andnon-functionalized SWNTs will be used as exemplary functionalized andnon-functionalized carbon nanomaterials. One skilled in the art willappreciate, however, that any suitable functionalized andnon-functionalized carbon nanomaterials may be used in lieu of, or inaddition to, f-SWNTs and non-functionalized SWNTs

Preferably, the permittivity attenuation layer 114 is interposed betweenthe inner-leg conductive trace of the differential pair and thesubstrate only in a length-mismatch region defined by the turn throughwhich the differential pair traverses, i.e., this length-mismatch regionextends from a location near where the length mismatch between theouter- and inner-leg conductive traces begins to a location near wherethe length mismatch ends. It is possible, therefore, for a conductivetrace that is routed through multiple turns (i.e., turns where theconductive trace is the inner-leg of the differential pair) to have itspermittivity attenuated at each such turn by a separate permittivityattenuation layer. Moreover, it is possible for both of the conductivetraces of a differential pair to have their permittivity attenuated in acase where the differential pair is routed through multiple turns (e.g.,one of the conductive traces is the inner-leg in a first turn and theother of the conductive traces is the inner-leg in a second turn).

In the embodiment shown in FIG. 1, the method 100 continues by laserablating the laminate core 102 and applying the permittivity attenuationlayer 114 containing f-SWNTs and, optionally, non-functionalized SWNTsto the laser ablated surface of the laminate core 102 exposed in theopen region 108 (step 110).

In the step 110, the laminate core 102 is laser ablated in the region108 to a suitable depth. One skilled in the art will appreciate that thedepth of laser ablation will vary depending on the application.Typically, it is desirable to laser ablate as deeply as possible intothe substrate without adversely impacting the substrate.

Also in step 110, f-SWNTs and, optionally, non-functionalized SWNTs maybe blended into a suitable resin carrier (e.g., uncured epoxy-basedresin) in an amount sufficient to achieve a suitable loading level, andthen deposited on the open region 108 to form a resin coating (i.e., thepermittivity attenuation layer 114, in an at least partially uncuredstate) via any number of conventional techniques well known in the art.Suitable techniques for applying the resin carrier having the f-SWNTsand the non-functionalized SWNTs (if any) blended therein include, butare not limited to, screen coating, spray-coating, and dip/immersioncoating. Preferably, the resin carrier, when cured, bonds to the varnishcoating of the laminate core 102.

Suitable resin carriers include, but are not limited to, an epoxy-basedresin, a bismaleimide triazine (BT) resin, a polyphenylene oxide(PPO)/triallylisocyanurate (TAIC) composition, and combinations thereof.

The loading level of the f-SWNTs and the non-functionalized SWNTs (ifany) in the resin carrier may be empirically determined based on theamount of permittivity attenuation/signal propagation delay that isdesired.

The resin carrier having the f-SWNTs and the non-functionalized SWNTs(if any) blended therein is applied to form a resin coating permittivityattenuation layer 114 having a suitable thickness. One skilled in theart will appreciate that the thickness of the permittivity attenuationlayer 114 will vary depending on the application. Typically, it isdesirable to form the permittivity attenuation layer 114 flush with thesurface of the substrate, in which case the thickness of thepermittivity attenuation layer 114 is defined by the depth of laserablation. Preferably, the width of the permittivity attenuation layer114 (which is defined by the region 108 opened up in the photoresist104) is selected to correspond to that of the inner-leg conductive trace120.

Stage 1(c) of FIG. 1 shows a partial, sectional view of the laminatecore 102 having the permittivity attenuation layer 114 applied theretoin the region 108 opened up in the photoresist 104.

The method 100 continues by curing the resin coating (i.e., thepermittivity attenuation layer 114, in an at least partially uncuredstate), and then stripping the photoresist 104 (step 115). The curingand stripping may be accomplished using any of numerous conventionaltechniques well known in the art. In the step 115, curing the resin inthe permittivity attenuation layer 114 serves both to bond thepermittivity attenuation layer 114 to the laminate core 102 and to bondthe functionalized SWNTs and non-functionalized SWNTs (if any) to thecore material.

Stage 1(d) of FIG. 1 shows a partial, sectional view of the laminatecore 102 and the permittivity attenuation layer 114, after thephotoresist has been stripped away.

The method 100 continues by laminating copper foil (116 in stage 1(e),described below) onto the laminate core 102 and the permittivityattenuation layer 114 (step 120). The copper lamination may beaccomplished using any of numerous conventional copper foil laminatingtechniques well known in the art.

Stage 1(e) of FIG. 1 shows a partial, sectional view of the laminatecore 102 and the permittivity attenuation layer 114, after the copperfoil 116 has been laminated thereto.

The method 100 continues by etching the copper foil (step 125). Thisforms the copper traces, i.e., the outer-leg conductive trace 118 on thelaminate core 102 and the inner-leg conductive trace 120 on thepermittivity attenuation layer 114. The copper etching may beaccomplished using any of numerous conventional copper etchingtechniques well known in the art.

Stage 1(f) of FIG. 1 shows a partial, sectional view of an enhancedlaminate core 117 that comprises the laminate core 102, the outer-legconductive trace 118 formed atop the laminate core 102, and theinner-leg conductive trace 120 formed in physical and electrical contactwith the permittivity attenuation layer 114. A portion of the routing ofthe outer-leg conductive trace 118 and a portion of the routing of theinner-leg conductive trace 120 are shown in dotted lines in stage 1(f).The outer-leg conductive trace 118 and the inner-leg conductive trace120 together define a portion of the differential pair 122.

Because the inner-leg conductive trace 120 is formed on and inelectrical contact with the permittivity attenuation layer 114, whichcontains f-SWNTs and, optionally, non-functionalized SWNTs, thepermittivity of the inner-leg conductive trace 120 can be attenuated tomatch that of the outer-leg conductive trace 118. This permittivitytailoring can be easily accomplished by, for example, altering theloading level of the f-SWNTs and the loading level of thenon-functionalized SWNTs (if any) in the resin carrier used to producethe permittivity attenuation layer 114. In this fashion, permittivitycan be tailored to the trace geometry to ensure that in-pair skew iseliminated, or at least substantially reduced.

The loading level of the f-SWNTs and the loading level of thenon-functionalized SWNTs (if any) in the resin carrier used to producethe permittivity attenuation layer may be empirically determined basedon the amount of permittivity attenuation/signal propagation delay thatis desired.

Subsequent to the step 125, the enhanced laminate core 117 may besubjected to conventional inner core processing steps well known in theart to complete the fabrication of the PWB. For example, a plurality ofcores, one or more of which correspond to the enhanced laminate core117, may be laminated together using partially cured varnish/glasslayers (typically in the form of one or more “prepregs”) withtemperature and pressure, causing the varnish coating to flow betweenthe layers to form a robust composite laminate structure. A sheet ofwhat is referred to as “prepreg” is a glass cloth impregnated with aresin solution which is either dried or at least partially cured.

FIG. 2 is a chemical reaction diagram showing an exemplary synthesis offunctionalized single-wall carbon nanotubes (f-SWNTs) in accordance withsome embodiments of the present invention. SWNTs can be easilyfunctionalized using the exemplary synthesis shown in FIG. 2, theexemplary synthesis shown in FIG. 3, or any of a myriad of othersynthetic procedures known to those skilled in the art. The reactionscheme shown in FIG. 2 has three steps.

In the first step of the reaction scheme shown in FIG. 2, SWNTs arefunctionalized to include carboxylic functional groups by reacting themin a mixture of nitric acid (HNO₃) and sulfuric acid (H₂SO₄) (1:3-3:1)at 140° C. for approximately 5 hours.

In the second step of the reaction scheme shown in FIG. 2, thefunctionalized SWNTs prepared in the first step are furtherfunctionalized to include chloro functional groups by reacting them withthionyl chloride (SOCl₂) at 80° C. for approximately 24 hours.

In the third step of the reaction scheme shown in FIG. 2, thefunctionalized SWNTs prepared in the second step are furtherfunctionalized to include either amine functional groups or alkoxyfunctional groups by reaction with a final reagent (e.g., either anamine (R—NH₂) or an alcohol (R—OH)). R may be any suitableorganofunctional group. R is preferably a matrix-reactive group thatincludes at least one moiety (e.g., a vinyl-, allyl-, amine-, amide-, orepoxy-containing moiety) capable of reacting with and bind to the resinmatrix. By judicious selection of the final reagent, the SWNTs can befunctionalized to bind to the resin matrix.

FIG. 3 is a chemical reaction diagram showing another exemplarysynthesis of functionalized single-wall carbon nanotubes (f-SWNTs) inaccordance with some embodiments of the present invention. As notedabove, SWNTs can be easily functionalized using the exemplary synthesisshown in FIG. 2, the exemplary synthesis shown in FIG. 3, or any of amyriad of other synthetic procedures known to those skilled in the art.The reaction scheme shown in FIG. 3 advantageously prepares the f-SWNTsunder solvent-free conditions using a single step.

In the reaction scheme shown in FIG. 3, SWNTs are functionalized using4-tert-R-aniline in excess isoamyl nitrite. These reaction componentsare heated to reflux at 80° C. for 2 hours. R may be Cl, Br, NO₂, or anysuitable organofunctional group. Suitable organofunctional groupsinclude, but are not limited to, tert-butyl and CO₂CH₃. The reactionscheme shown in FIG. 3 is based on the procedure disclosed by Dyke etal., “Solvent-Free Functionalization of Carbon Nanotubes,” J. Am. Chem.Soc., 2003, Vol. 125, No. 5, pp 1156-1157, which is hereby incorporatedherein by reference in its entirety. In accordance with some embodimentsof the present invention, R is preferably a matrix-reactive group thatincludes at least one moiety (e.g., a vinyl-, allyl-, amine-, amide-, orepoxy-containing moiety) capable of reacting with and bind to the resinmatrix. By judicious selection of the final reagent, the SWNTs can befunctionalized to bind to the resin matrix.

Prophetic Example Functionalization

In this prophetic example, f-SWNTs are prepared under solvent-freeconditions. HiPco SWNTs (Unidym Inc., Sunnyvale, Calif., USA) arefunctionalized using 4-tert-butylaniline (2.5 equiv to SWNT carbon) inexcess isoamyl nitrite. These reaction components are heated to refluxat 80° C. for 2 hours. After reaction, the reaction product is cooled toroom temperature (r.t.), followed by purification using techniques wellknown in the art.

Blending.

In this prophetic example, an epoxy resin formulation is used. Oneskilled in the art will appreciate, however, that any suitable resin maybe used in lieu of, or in addition to, the particular epoxy resinformulation used in this prophetic example. The epoxy resin formulationused in the prophetic example includes: EPIKOTE Resin 828LVEL (availablefrom Momentive Specialty Chemicals, Inc., Columbus, Ohio) (100 parts);dicyandiamide (5 parts); and 2-ethyl-4-methylimidazole (1 part). EPIKOTEResin 828LVEL is a bisphenol-A type epoxy resin produced from bisphenolA and epichlorohydrin. Dicyandiamide and 2-ethyl-4-methylimidazole arecommonly used curing agents.

Also, in this prophetic example, the total weight percent of SWNTs(f-SWNTs alone, non-functionalized SWNTs alone, or both) included in theresulting composite is kept constant at approximately 1 wt %. Oneskilled in the art will appreciate, however, that the total weightpercent of SWNTs may be any suitable value (e.g., 0.1 wt % to 25 wt %).Preferably, the total weight percent of the SWNTs is selected toaccommodate an anticipated range of permittivity attenuation/signalpropagation delay (i.e., from a minimum anticipated permittivityattenuation/signal propagation delay to a maximum anticipatedpermittivity attenuation/signal propagation delay) that will be requiredof different permittivity attenuation layers. The total weight percentof SWNTs may be, for example, empirically determined based on such ananticipated range of permittivity attenuation/signal propagation delay.Working within the 1 wt % total weight percent of SWNTs of thisprophetic example, the individual loading levels of the f-SWNTs (0 wt %to 1 wt %) and the non-functionalized SWNTs (1 wt % to 0 wt %) in theresin matrix of a permittivity attenuation layer may be empiricallydetermined based on a particular amount of permittivityattenuation/signal propagation delay that is required of a particularpermittivity attenuation layer.

Also, in this prophetic example, chloroform is used as a solvent inwhich to disperse the SWNTs. Chloroform is merely an exemplary solventthat is suitable for use with an epoxy resin. One skilled in the artwill appreciate that any suitable solvent may be used in lieu of, or inaddition to, chloroform. Solvents suitable for use with epoxy resinsinclude, but are not limited to, chloroform, xylene, n-butanol, toluene,THF, and combinations thereof.

In this prophetic example, for each of several samples, 25 mg totalcarbon weight of SWNTs (f-SWNTs alone, non-functionalized SWNTs, orboth) are dispersed into a minimal amount of chloroform by bathsonication. The loading level of the f-SWNTs relative to the loadinglevel of the non-functionalized SWNTs in each sample is expressed as aratio. For example, in a 1:0 sample of this prophetic example, some ofthe f-SWNTs (25 mg) prepared in the functionalization step are dispersedin the chloroform. In a 1:1 sample of this prophetic example, some ofthe f-SWNTs (12.5 mg) prepared in the functionalization step and HiPcoSWNTs (Unidym Inc., Sunnyvale, Calif., USA) (12.5 mg) are dispersed inthe chloroform. In a 0:1 sample of this prophetic example, HiPco SWNTs(Unidym Inc., Sunnyvale, Calif., USA) (25 mg) are dispersed in thechloroform. This chloroform mixture is then combined with EPIKOTE Resin828LVEL (2.50 g). Then, the chloroform solvent is evaporated from themixture with continuous stirring. This evaporation is readilyaccomplished (in a fume hood with efficient ventilation) by directing astream of air over the continuously stirring mixture. The mixture isthen heated to 60° C. for 2 hours in a vacuum oven to remove anyremaining chloroform solvent. Then, dicyandiamide (125 mg) and2-ethyl-4-methylimidazole (25 mg) are added as curing agents, and theresulting epoxy resin formulation with the SWNTs blended therein isthoroughly stirred.

Applying, Curing, etc.

The epoxy resin formulation with the SWNTs blended therein is applied toa laminate core via screen coating to form a resin coating (i.e., step110 of method 100 shown in FIG. 1, described above). The resin coatingis initially cured for 1 hour at 100° C., and subsequently cured for 1hour at 150° C. to form the permittivity attenuation layer (i.e., step115 of method 100 shown in FIG. 1, described above). A copper trace issubsequently formed atop the permittivity attenuation layer usingconventional copper foil laminating and etching techniques well known inthe art (i.e., steps 120 and 125 of method 100 shown in FIG. 1,described above). The 1:0 sample produces the smallest amount ofpermittivity attenuation/signal propagation delay. The 1:1 sampleproduces an intermediate amount of permittivity attenuation/signalpropagation delay. The 0:1 sample produces the largest amountpermittivity attenuation/signal propagation delay. ***End of PropheticExample.***

FIG. 4 shows a block diagram of an exemplary design flow 400 used, forexample, in printed wiring board (PWB), laminate subcomposite,interconnect substrate, or semiconductor IC logic design, simulation,test, layout, and manufacture. Design flow 400 includes processes,machines and/or mechanisms for processing design structures or devicesto generate logically or otherwise functionally equivalentrepresentations of the design structures and/or devices described aboveand shown in FIG. 1. The design structures processed and/or generated bydesign flow 400 may be encoded on machine-readable transmission orstorage media to include data and/or instructions that when executed orotherwise processed on a data processing system generate a logically,structurally, mechanically, or otherwise functionally equivalentrepresentation of hardware components, circuits, devices, or systems.Machines include, but are not limited to, any machine used in a printedwiring board, laminate subcomposite, interconnect substrate, or ICdesign process, such as designing, manufacturing, or simulating acircuit, component, device, or system. For example, machines mayinclude: lithography machines, machines and/or equipment for generatingmasks (e.g. e-beam writers), computers or equipment for simulatingdesign structures, any apparatus used in the manufacturing or testprocess, or any machines for programming functionally equivalentrepresentations of the design structures into any medium (e.g. a machinefor programming a programmable gate array).

Design flow 400 may vary depending on the type of representation beingdesigned. For example, a design flow 400 for building an applicationspecific IC (ASIC) may differ from a design flow 400 for designing astandard component or from a design flow 400 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 4 illustrates multiple such design structures including an inputdesign structure 420 that is preferably processed by a design process410. Design structure 420 may be a logical simulation design structuregenerated and processed by design process 410 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 420 may also or alternatively comprise data and/or programinstructions that when processed by design process 410, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 420 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 420 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 410 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIG. 1. As such,design structure 420 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 410 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,apparatus, devices, or logic structures shown in FIG. 1 to generate anetlist 480 which may contain design structures such as design structure420. Netlist 480 may comprise, for example, compiled or otherwiseprocessed data structures representing a list of wires, discretecomponents, logic gates, control circuits, I/O devices, models, etc.that describes the connections to other elements and circuits in aprinted wiring board, laminate subcomposite, interconnect substrate, orintegrated circuit design. Netlist 480 may be synthesized using aniterative process in which netlist 480 is resynthesized one or moretimes depending on design specifications and parameters for the device.As with other design structure types described herein, netlist 480 maybe recorded on a machine-readable data storage medium or programmed intoa programmable gate array. The medium may be a non-volatile storagemedium such as a magnetic or optical disk drive, a programmable gatearray, a compact flash, or other flash memory. Additionally, or in thealternative, the medium may be a system or cache memory, buffer space,or electrically or optically conductive devices and materials on whichdata packets may be transmitted and intermediately stored via theInternet, or other networking suitable means.

Design process 410 may include hardware and software modules forprocessing a variety of input data structure types including netlist480. Such data structure types may reside, for example, within libraryelements 430 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, and the like). The data structure types may furtherinclude design specifications 440, characterization data 450,verification data 460, design rules 470, and test data files 485 whichmay include input test patterns, output test results, and other testinginformation. Design process 410 may further include, for example,standard mechanical design processes such as stress analysis, thermalanalysis, mechanical event simulation, process simulation for operationssuch as laminating, casting, molding, die press forming, and the like.One of ordinary skill in the art of mechanical design can appreciate theextent of possible mechanical design tools and applications used indesign process 410 without deviating from the scope and spirit of theinvention. Design process 410 may also include modules for performingstandard circuit design processes such as timing analysis, verification,design rule checking, place and route operations, and the like.

Design process 410 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 420 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 490.Design structure 490 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in a IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 420, design structure 490 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIG. 1. In one embodiment, design structure 490 maycomprise a compiled, executable HDL simulation model that functionallysimulates the devices shown in FIG. 1.

Design structure 490 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 490 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIG. 1. Design structure 490may then proceed to a stage 495 where, for example, design structure490: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, and the like.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. For example, although someembodiments of the present invention are described herein in the contextof a printed wiring board (PWB), the present invention may be utilizedin the context of other substrates, such as a laminate subcomposite, aninterconnect substrate (e.g., an interposer or a module substrate), aflex cable, or an IC. Thus, while the present invention has beenparticularly shown and described with reference to some embodimentsthereof, it will be understood by those skilled in the art that theseand other changes in form and detail may be made therein withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. An apparatus, comprising: a substrate; a firstconductive trace; a permittivity attenuation layer interposed betweenthe first conductive trace and the substrate, wherein the permittivityattenuation layer comprises a resin matrix containing functionalizedcarbon nanomaterial.
 2. The apparatus as recited in claim 1, furthercomprising a second conductive trace, wherein the first conductive tracecomprises an inner-leg conductive trace of a differential pair and thesecond conductive trace comprises an outer-leg conductive trace of thedifferential pair.
 3. The apparatus as recited in claim 2, wherein thefunctionalized carbon nanomaterial comprises functionalized single-wallcarbon nanotubes (f-SWNTs), and wherein the loading level of the f-SWNTsis selected to attenuate the permittivity of the inner-leg conductivetrace to match the permittivity of the outer-leg conductive trace. 4.The apparatus as recited in claim 1, wherein the permittivityattenuation layer further comprises non-functionalized carbonnanomaterial blended with the functionalized carbon nanomaterial in theresin matrix.
 5. The apparatus as recited in claim 4, further comprisinga second conductive trace, wherein the first conductive trace comprisesan inner-leg conductive trace of a differential pair and the secondconductive trace comprises an outer-leg conductive trace of thedifferential pair.
 6. The apparatus as recited in claim 5, wherein thefunctionalized carbon nanomaterial comprises functionalized single-wallcarbon nanotubes (f-SWNTs), wherein the non-functionalized carbonnanomaterial comprises non-functionalized single-wall carbon nanotubes(SWNTs), and wherein the loading level of the f-SWNTs and the loadinglevel of the non-functionalized SWNTs are selected to attenuate thepermittivity of the inner-leg conductive trace to match the permittivityof the outer-leg conductive trace.
 7. The apparatus as recited in claim1, wherein the substrate is a laminate core for use in a printed wiringboard (PWB), wherein the laminate core comprises a glass fiber substrateimpregnated with a varnish coating, and wherein the resin matrix of thepermittivity attenuation layer and the varnish coating of the laminatecore each includes an epoxy-based resin.
 8. The apparatus as recited inclaim 7, further comprising a second conductive trace, wherein the firstconductive trace comprises an inner-leg conductive trace of adifferential pair and the second conductive trace comprises an outer-legconductive trace of the differential pair.
 9. The apparatus as recitedin claim 8, wherein the functionalized carbon nanomaterial comprisesfunctionalized single-wall carbon nanotubes (f-SWNTs), and wherein theloading level of the f-SWNTs is selected to attenuate the permittivityof the inner-leg conductive trace to match the permittivity of theouter-leg conductive trace.
 10. An apparatus, comprising: a substrate; afirst conductive trace; a permittivity attenuation layer interposedbetween the first conductive trace and the substrate, wherein thepermittivity attenuation layer comprises a resin matrix containingfunctionalized carbon nanomaterial, and wherein the functionalizedcarbon nanomaterial comprises functionalized single-wall carbonnanotubes (f-SWNTs) functionalized to include either amine functionalgroups or alkoxy functional groups.
 11. The apparatus as recited inclaim 10, wherein the permittivity attenuation layer further comprisesnon-functionalized carbon nanomaterial blended with the functionalizedcarbon nanomaterial in the resin matrix, and wherein thenon-functionalized carbon nanomaterial comprises non-functionalizedsingle-wall carbon nanotubes (SWNTs).
 12. A method in a computer-aideddesign system for generating a functional design model of an apparatus,the method comprising: generating a functional design model of asubstrate; generating a functional design model of a first conductivetrace; generating a functional design model of a permittivityattenuation layer interposed between the first conductive trace and thesubstrate, wherein the permittivity attenuation layer comprises a resinmatrix containing functionalized carbon nanomaterial.
 13. The method asrecited in claim 12, further comprising: generating a functional designmodel of a second conductive trace, wherein the first conductive tracecomprises an inner-leg conductive trace of a differential pair and thesecond conductive trace comprises an outer-leg conductive trace of thedifferential pair.
 14. The method as recited in claim 13, wherein thefunctionalized carbon nanomaterial comprises functionalized single-wallcarbon nanotubes (f-SWNTs), and wherein the loading level of the f-SWNTsis selected to attenuate the permittivity of the inner-leg conductivetrace to match the permittivity of the outer-leg conductive trace. 15.The method as recited in claim 12, wherein the permittivity attenuationlayer further comprises non-functionalized carbon nanomaterial blendedwith the functionalized carbon nanomaterial in the resin matrix.
 16. Themethod as recited in claim 15, further comprising; generating afunctional design representation of a second conductive trace, whereinthe first conductive trace comprises an inner-leg conductive trace of adifferential pair and the second conductive trace comprises an outer-legconductive trace of the differential pair.
 17. The method as recited inclaim 16, wherein the functionalized carbon nanomaterial comprisesfunctionalized single-wall carbon nanotubes (f-SWNTs), wherein thenon-functionalized carbon nanomaterial comprises non-functionalizedsingle-wall carbon nanotubes (SWNTs), and wherein the loading level ofthe f-SWNTs and the loading level of the non-functionalized SWNTs areselected to attenuate the permittivity of the inner-leg conductive traceto match the permittivity of the outer-leg conductive trace.
 18. Themethod as recited in claim 12, wherein the substrate is a laminate corefor use in a printed wiring board (PWB), wherein the laminate corecomprises a glass fiber substrate impregnated with a varnish coating,and wherein the resin matrix of the permittivity attenuation layer andthe varnish coating of the laminate core each includes an epoxy-basedresin.
 19. The method as recited in claim 18, further comprising:generating a functional design representation of a second conductivetrace, wherein the first conductive trace comprises an inner-legconductive trace of a differential pair and the second conductive tracecomprises an outer-leg conductive trace of the differential pair. 20.The method as recited in claim 19, wherein the functionalized carbonnanomaterial comprises functionalized single-wall carbon nanotubes(f-SWNTs), and wherein the loading level of the f-SWNTs is selected toattenuate the permittivity of the inner-leg conductive trace to matchthe permittivity of the outer-leg conductive trace.